1) Field of the Invention
The present invention relates to an activation system for a passenger protection apparatus which is made to activate the passenger protection apparatus in accordance with a behavior of a vehicle.
2) Description of the Related Art
So far, for example, in an air bag system for use in a vehicle, an acceleration of the vehicle is detected through the use of an acceleration sensor in an air bag ECU located at a central portion of the vehicle in its longitudinal directions to make a decision indicating the occurrence of a collision of the vehicle when a result of the acceleration detection exceeds a predetermined threshold so that an air bag is deployed for protecting a passenger(s) (including a driver) from the impact due to the collision. Moreover, in such an air bag system for a vehicle, a mechanical acceleration switch, serving as a collision sensor, is provided in a front portion of the vehicle to transmit a switching signal to the air bag ECU when detecting a collision acceleration above a predetermined value. Still moreover, the air bag ECU changes the above-mentioned predetermined threshold to a value lower than a normal value on the basis of the switching signal, thereby detecting the impact at the front portion of the vehicle securely to spread the air bag.
However, such a mechanical acceleration switch merely outputs a switching signal indicative of only the presence or absence of the collision and this signifies that difficulty is experienced in acquiring the collision acceleration in detail. On the other hand, in place of the mechanical acceleration switch, there has been proposed a technique in which an electronic acceleration sensor is provided in a front portion of a vehicle so that a detailed collision acceleration is detected on the basis of an analog signal from the electronic acceleration sensor.
There is a problem which arises with the employment of such an electronic acceleration sensor functioning as a collision sensor, however, in that noise acting on a communication line between the acceleration sensor and the air bag ECU can considerably throw off the analog acceleration signal to be transmitted from the acceleration sensor.
In consideration of this problem, PCT Japan National Publication No. 2001-515815 discloses a technique in which an evaluation function is provided in a collision sensor unit to make a decision on the occurrence of collision and, when the evaluation function detects and confirms the fact that a collision sensor signal forming a analog signal or a collision signal derived from the collision sensor signal reaches a predetermined value within a predetermined period of time, a code signal is transmitted to an air bag ECU in the form of a digital signal.
However, in the case of the technique disclosed in PCT Japan National Publication No. 2001-515815, because of the collision decision function being provided in the collision sensor unit, in addition to an CPU of the air bag ECU, there is a need to use another CPU in the collision sensor unit, which complicates the system configuration and increases the manufacturing cost.